1. Field of the Invention
The present invention relates to a method for fabricating composite ceramic structures, and more particularly, the present invention relates to a method for fabricating multi-phase ceramic filaments to be used as building blocks for the structures.
2. Background of the Invention
When monolithic ceramic structures are stressed to the point of failure, they often fail with little or no warning, i.e., catastrophically. These monolithic ceramics simply load elastically to a maximum stress and then fail all at once.
Several techniques for improving the toughness of ceramic structures have been attempted. For example, fibers have been added to bulk ceramic material to increase its toughness. These are called continuous fiber ceramic composites. However, even these tougher materials can fail catastrophically.
Attempts have been made at forming ceramic structures which fail xe2x80x9cgracefullyxe2x80x9d (i.e., with warning). One of these structures are known as fibrous monoliths (FMs), and are fabricated from billets comprising a composite ceramic containing both a strong cellular phase (a core) surrounded by a phase (in the form of a sleeve) designed to dissipate energy during fracture. Current FM production processes utilize multi-step protocols and heterogeneous materials. These processes make use of ram extrusion of slugs, and are thus batch processes by definition.
U.S. Pat. No. 4,772,524 awarded to Coblenz on Sep. 20, 1988 discloses a fibrous monolith whereby a cotton thread runs co-axially with the monolith produced. U.S. Pat. No. 5,645,781 awarded to Popovic et al. on Jul. 8, 1997 discloses a method for clamping or wrapping a sleeve of ceramic material around a dense core of ceramic material. These processes require separate steps and are thus considered xe2x80x9cbatchxe2x80x9d operations.
Another drawback to current FM production processes is that hot pressing (often in an inert atmosphere) is generally used to densify the materials after their formation. To date, hot pressing in an inert atmosphere has been used to densify FMs such as silicon nitride (cell)/boron nitride (cell boundary) systems and diamond-cobalt (cell)/cobalt (cell boundary) systems. These FMs have been made from thermoplastic co-polymer blends that also require special processing during extrusion, such as close control of pressure and need for use of elevated temperatures above 160xc2x0 C. Hot pressing limits the shapes of the FMs that can be produced. Also, this process is costly.
A need exists in the art for a process for producing ceramic composites which utilize common extrusion equipment and similar phase materials. The process should produce a structure which yields gracefully. The process also should utilize common ceramic materials and sintering steps conducted in air and at ambient pressure to further minimize cost. Finally, the process should incorporate the least number of steps, and preferably comprise a single step or continuous process so as to expedite production in large industrial scale scenarios.
It is an object of the present invention to provide a method for producing ceramic composite materials that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a continuous process for producing ceramic composite material. A feature of the invented method is the use of a standard extrusion machine. An advantage of the invented method is that minimal fabricator training is required, resulting in an optimization of personnel and existing equipment, and ultimately lower costs.
Yet another object of the present invention is to provide a process for producing robust unidirectional or multidirectional fibrous monoliths. A feature of the invented method is careful matching of shrinkages and thermal-expansion coefficients of the various phases comprising the monoliths. An advantage of the invented method is the ability to sinter in air and at atmospheric pressure (i.e., pressure-less sintered) to produce the fibrous monoliths, thereby obviating the need for hot pressing in an inert atmosphere to accomplish densification. This leads to a reduction in cost of many FM parts by a factor of up to 100.
Still another object of the present invention is to provide a process for producing fibrous monoliths which are stable at a myriad of operating conditions. A feature of the process is the utilization of different particles sizes of the same compound for both the cell phase and the cell boundary layer. An advantage of the invented process is the elimination of material compatibility problems, thereby also eliminating diffusion between phases so as to permit operation at higher temperatures. Stability in oxidizing, inert and reducing temperatures also is realized.
Briefly, the invention provides for a method for producing composite ceramics, the method comprising simultaneously forming a ceramic core coaxial with a ceramic sleeve.